HEPA (high efficiency particulate air) filters are well known in the art and are widely used in those industries, clean rooms and the like wherein the highest quality clean air is required. Most clean rooms utilize a vertical air flow system with HEPA filters arranged in a grid in the ceiling. Air is appropriately ducted to the HEPA filters above the ceiling and clean air is flooded into the area of the clean room from the HEPA filters and passes down through the floor. A Class 100 clean room is a clean room having 100 airborne particles per cubic foot of air. Class 10 (or less) clean rooms are presently being contemplated.
Some of the most stringent requirements tday are to be found in the semi-conductor industry. Until recently, the goal was to remove particles down to 0.3 micrometer in size. With the advent of greatly reduced size integrated circuits, particle sizes less than 0.1 micrometer have become of interest.
The usual HEPA filter, having the best grade filtering medium is generally characterized by a collection efficiency of about 99.97% for 0.3 micrometer particles and is further characterized by a pressure drop of 20 mm H.sub.2 O at a standard air velocity of 2.5 cm/s perpendicular to the filter medium. Another type of HEPA filter provided with a different filter medium is generally characterized by a collection efficiency of 94% for 0.3 micrometer size particles and a pressure drop of 10 mm H.sub.2 O at a standard air velocity. The lowest grade filter of HEPA construction is characterized by a collection efficiency of only about 30% for 0.3 micrometer size particles, and a low pressure drop of 2 mm H.sub.2 O at standard air velocity. In general, conventional HEPA filters do not collect 0.1 micrometer particles at an efficiency that is satisfactory for very low particle count clean rooms.
An electrostatically augmented HEPA filter is described in U.S. Pat. No. 4,357,150. This patent teaches the combination of an electrostatically augmented HEPA filter and an ionizer located at the upstream or air inlet face thereof. The ionizer comprises alternate plates and wires in parallel spaced relationship. The wires are connected to a high d.c. voltage source. The plates are connected to ground. The ionizer section charges the particulate material entering the HEPA filter. Those spacers of the HEPA filter having an exposed longitudinal edge at the air inlet side of the HEPA filter are connected to ground. Those spacers having an exposed longitudinal edge at the air discharge face of the HEPA filter are connected to a high voltage d.c. electric power source.
It has been demonstrated that a filter device of the type taught in U.S. Pat. No. 4,357,150 is characterized by a dramatic increase in the efficiency of the HEPA filter with a simultaneous reduction in the rate of increase of pressure drop as the HEPA filter becomes dirty. This, in turn, results in an extension of the life of the HEPA filter with significant savings in material and labor to replace and test a new HEPA filter for proper edge seal and the like.
As reported by Senichi Masuda and Naoki Sugita in the paper ELECTROSTATICALLY AUGMENTED AIR FILTER FOR PRODUCING ULTRA CLEAN AIR presented at the 74th Annual Meeting of the Air Pollution Control Association, Philadelphia, Pa., June 21-26, 1981, tests were made of three electrostatically augmented air filter devices of the type taught in U.S. Pat. No. 4,357,150, utilizing filtering media of the 99.97%, 94% and 30% efficiency types. Following the teachings of U.S. Pat. No. 4,357,150, the electrostatically enhanced HEPA filter utilizing the 99.97% medium demonstrated a collection efficiency for 0.15 micrometer size particles of about 99.9998%. Utilizing the 94% medium (at a lower cost and a smaller pressure drop) a collection efficiency for 0.15 micrometer size particles as high as 99.999% was demonstrated. For a 30% medium, with a very low pressure drop, a collection efficiency of 97% was obtained for 0.3 micrometer particles, which is more than adequate for many usual air cleaning purposes.
Despite its increased efficiency and reduction in pressure drop as the filter medium gets dirty, acceptance of the electrostatically augmented HEPA filter has been slow because of concern that arcing might occur between adjacent spacers. For a nominal 24.times.24.times.12 inch HEPA filter, the capacitance of the aluminum spacers would produce about 0.104 mJ of energy in an arc, which is about the minimum ignition energy for explosive gases. The possibility of a fire is another concern. Furthermore, there is a risk of holes being formed in the filter medium and particulate matter being generated during the arcing from the charged spacer through the filter medium to the adjacent grounded spacer.
Since, in such an electrostatically augmented filter device all of the spacers exposed at the air inlet face of the filter are connected together and to ground, and since all of the spacers exposed on the air discharge face of the filter are connected together and to a source of high voltage, should an arc discharge occur at a part of a certain spacer, the charges of all of the spacers move to the discharging part, not only to increase the discharge energy, but also to create a temporary drop of spacer voltage and a temporary reduction of dust collecting efficiency. In addition, as indicated above, the filter medium may become damaged or perforated. Furthermore, when the ambient humidity is high, the non-conductive characteristic of the filter medium is weakened, reducing dust collecting efficiency.
These problems are addressed in U.S. Pat. No. 4,509,958. Among other things, this reference teaches the connection together of those spacers exposed at the air inlet face of the filter and the connection together of those spacers exposed at the air discharge face of the filter by various embodiments utilizing an electroconductive material having an electric resistance for preventing movement of charges on the spacers so that, should arcing occur at a certain spacer, the amount of discharge is restricted and charges of the other spacers are greatly restricted from moving to the discharge point, minimizing the discharge energy. This reference further contemplates the incorporation of insulating material between the filter medium and the spacers exposed at the air inlet face of the filter and/or between the filter medium and the spacers exposed at the air discharge face of the filter, to minimize arcing and the effects of high humidity.
The solutions to the above outlined problems, as presented in the above noted U.S. Pat. No. 4,509,958 require major modification of the conventional HEPA construction. The present invention is based upon the discovery that the above noted problems can be overcome, while still achieving the dramatic improvements demonstrated by an electrostatically augmented HEPA filter by minimum modification to the HEPA filter construction. The teachings of the present invention assure that, should an arc occur, the energy thereof is limited to the extent that damage cannot result. Furthermore, the modifications to a conventional HEPA filter, according to the present invention, are such that retrofitting of existing HEPA filters can readily be accomplished.